Safari 5.0.1 is available from Apple as a free download for Windows and for Mac OS X (Leopard or better). Mac users can also find it in Software Update.

This is an incremental upgrade, but it comes with one big new feature: Safari now has a real platform for third-party extensions, a feature that Firefox and Chrome have had for some time.

Safari 5 arrived in early June, and in addition to dozens of other enhancements (including the much-discussed Reader feature) it included a new architecture for creating lightweight browser extensions that enhance and personalize web pages and web services. Wednesday’s update now lets you install and run those extensions. Apple has also launched a new Extensions Gallery where you can browse the available extensions and download them.

All the major browsers — Safari included — have had a variety of plug-ins, add-ons and toolbars available for years. But Safari’s new extension architecture is much closer to the format recently adopted by Google Chrome and Firefox. This new breed of extensions can be written using HTML, JavaScript, CSS and other web standards. It makes for a much gentler learning curve for potential developers, and for an experienced web programmer, the effort required to create and distribute a standards-based extension is almost trivial. For users, these extensions are easier to maintain and less likely to slow down the browser.

Mozilla calls its lightweight extension project Jetpack, and it’s being incorporated into the newest Firefox releases. The next version of Google’s browser will let users sync their extensions across multiple installations of the browser.

Go to extensions.apple.com to see the gallery of extensions being promoted by Apple. Also, keep in mind that anyone can create and distribute a Safari extension, so distribution isn’t controlled like the App Store. For safety’s sake, Safari extensions are sandboxed inside the browser and signed with a digital certificate so you know you’re getting updates from the same person who created the original.

Apple is promoting a few big-name creations in the gallery. There’s an official Twitter extension, which integrates a simple toolbar Twitter client into your browser, one from MLB that displays scores and headlines, and an eBay manager sidebar for keeping a close eye on your auctions. There’s one on the way from Instapaper.

Of course, the irreverent extensions are more interesting. There’s Defacer, which hides “Like” buttons and other Facebook cruft you find around the web. Shut Up hides comments by default on blogs. A Cleaner YouTube removes visual distractions from video pages, promising to turn YouTube into “a clean and tranquil place” as if that’s even remotely possible.

There are around 100 extensions to choose from right now, and since the new extensions framework in Safari is so simple to develop for, we expect the list to keep growing quickly.

There is one other notable safety enhancement to Safari 5.0.1 — the form auto-fill vulnerability has been patched. This fixes a vulnerability that hackers could exploit to grab personal information from a user by forcing the browser to auto-fill a hidden web form with locally stored data. So, even if you may not care for extensions, you should upgrade Safari for this reason alone.

People who like to run pre-release versions of browsers in order to access the latest features have a new choice: Google Chrome Canary.

Canary has all the bleeding-edge features found in the developer and beta releases of Google Chrome. But unlike the other channel releases, Chrome Canary allows you to run the pre-release browser without overwriting other installations of Chrome on the same system. So, you can now run a regular version of Chrome and a pre-release, auto-updating version of Chrome on the same computer at the same time.

You can download Chrome Canary today, but it is a Windows-only release for now. We expect Google to follow with canaries for other operating systems soon.

Early adopters — mostly curious geeks and developers working with the latest web standards — prefer to run beta versions of browsers. Beta testing allows them to gain intimate first-hand knowledge of the new capabilities that will be found in the next versions of each browser. But beta versions and regular versions of the same browsers both access the same file resources on your computer, a restriction that prevents you from running two different versions side-by-side. Try launching a Firefox 4 beta while Firefox 3.x is open. You’ll see an error: “Only one copy of Firefox can be open at a time.”

On the fence about running an unstable pre-release browser? Canary can help you take the plunge safely.

Chrome Canary side-steps this issue. As Google engineer Huan Ren explains on the Chromium-dev list, “the installer will install Google Chrome canary build to a separate directory with different default user profile, short cuts, and icons, i.e. everything should be separate from existing Google Chrome installation.”

With this release, there are now four versions of Chrome available. The others are “dev,” the least stable build intended for developers, “beta,” which is more stable than dev but not fully baked, and the regular Chrome release, the rigorously-tested version that’s the default option for the public.

On the same developer’s e-mail list, Google’s Mark Larson says Canary will be the most bleeding-edge of all Chrome builds. It will auto-update more frequently than any of the other versions available to developers.

“The canary usually updates more frequently than the Dev channel (higher risk of bustage), and we’re working on making it update as often as we have successful nightly builds. When something doesn’t work on the canary, I can just fall back to my Beta Google Chrome,” he writes.

Hence the name “Canary” — a reference to the canary in the coal mine. Google recently announced it would be speeding up the Chrome development cycle to push major milestone releases more often. This increased velocity means it will need to begin testing new features in the wild sooner and collecting feedback more quickly.

Giving users the option to run a more advanced version of Chrome without having to fully commit to the dangerous lifestyle of an alpha tester should help increase the number of people willing to test the new browser.

Chrome Canary also has a different, all-yellow icon — instead of the multi-colored Chrome icon or the all-blue Chromium icon — so it’s easy to spot on your desktop. The beta, dev and stable channel builds of Chrome all use the same familiar rainbow icon. Also, the skin of the browser is blue, helping you tell it apart from other versions of Chrome, which use the same gray skin.

Inspired by Apple’s Automator application, Fake looks like a combination of Safari and Automator and allows you to run (and re-run) “fake” interactions with the web.

Power Users will love Fake for automating tedious web tasks like filling out lengthy forms and capturing screenshots. Developers can use Fake for graphically configuring automated tests for their web apps, including assertions.

Like Ditchendorf’s other projects, Fake is based on WebKit and his own ParseKit. It uses AppleScript for the web automations. For now, it’s available as a time-limited demo that expires at the end of July.

You can download it and run it on any modern Linux installation. It seems to run on Ubuntu just fine, though PNG support is a little wonky. The good news is that the folks on Github are actively submitting patches.

Mosaic was the first graphical web browser. It was born in the early ’90s, created by a small team headed by Marc Andreessen. The same crew would go on to found Netscape Communications and build the Netscape browser, which would eventually lead to the Mozilla browser, and finally to Firefox. So, running Mosaic is basically taking the browsing experience all the way back to its roots.

Dipert acknowledges the work of two other coders who ported the old code to run on the modern Linux: Sean MacLennan and Alan Wylie. As MacLennan says on his site, “If you are going to run a 10-year old protocol (HTML), you might as well use a 10-year old browser.”

I first started using Mosaic at the beginning, in 1993. We had it running at my college radio station, and we DJs would use it to download the news wires we’d read on air at the top of every hour. I also used it to browse Wired’s gopher server and read the magazine articles on my computer in my dorm room. About two years later, HotWired arrived on the web proper, and I used Mosaic to browse it.

Opera has pushed out a pre-alpha build of the next version of its flagship desktop web browser. For Opera 10.5, as the next version will be known, the focus is on speed, and while this pre-alpha release is a long way from done, the speed boost is already noticeable.

This pre-alpha release is currently only available for Windows and Mac OS X users. Opera says a Linux version will be released soon. If you’d like to test out Opera 10.5, download links can be found at the bottom of the Opera Labs announcement page.

Much of Opera 10.5′s speed improvements come from the revamped JavaScript engine, known as Carakan, which Opera claims is up to seven times faster than the engine in the current shipping version, Opera 10. It’s worth noting that the Mac version is not nearly as far along as the Windows release, so Mac users may not notice a dramatic speed boost in this early release.

Also new under the hood is support for CSS3 transitions and transforms, which means that the cool CSS 3 transform tricks we told you about last week will work in this version of Opera (note that you’ll need to add the Opera flag to your CSS code, for example, -o-transition-property).

The pre-alpha release of Opera 10.5 also features a new graphics engine that can take advantage of hardware acceleration (when it’s available) to render SVG graphics. Given the possibility of very complex graphics thanks to HTML5′s canvas tag, we expect to see more emphasis on graphics engine in the coming year (think of hardware-accelerated graphics as a sequel to the JavaScript engine contests of recent months).

Opera is also one of the last browsers to jump on the private-browsing-mode bandwagon, but it is finally here in this release.

While the speed boosts in Opera 10.5 are noticeable, particularly on JavaScript-intensive sites and web apps, Opera 10.5 is very much a work in progress and lacks some very basic features — like printing in the Mac version. Also missing is support for Opera’s Unite web server tools.

As you would expect from the pre-alpha designation Opera 10.5 is also somewhat unstable, but if you’d like to test out the latest release, head over to the Opera Labs page and grab a copy.